Notes on Membrane Structure and Function

The plasma membrane separates the living cell from its nonliving surroundings

• this thin barrier controls traffic into and out of the cell

• like other membranes, the plasma membrane is selectively permeable, allowing some substances to cross more easily than others

Membrane Structure

• the main macromolecules in membranes are lipids and proteins, but include some carbohydrates

• the most abundant lipids are phospholipids

• the phospholipids and proteins in membranes are described by the fluid mosaic model

• the molecules in the bilayer are arranged such that the hydrophobic fatty acid tails are sheltered from water while the hydrophilic phosphate groups interact with water

Phospholipids and most other membrane constituents are amphipathic molecules which have both hydrophilic and hydrophobic regions

Membranes are Fluid

• membrane molecules are held in place by relatively weak hydrophobic interactions

• most of the lipids and some proteins can drift laterally in the plane of the membrane, but rarely flip-flop from one layer to the other

Lateral movement(107 times per second)

Flip-flop( once per month)

Membranes are Mosaics of Structure and Function

• proteins determine most of the membrane’s specific functions

Membrane carbohydrates are important for cell-cell recognition

• the membrane plays the key role in cell-cell recognition

• cell-cell recognition is the ability of a cell to distinguish one type of neighboring cell from another

• this attribute is important in cell sorting and organization into tissues and organs during development

• it is also the basis for rejection of foreign cells by the immune system

• cells recognize other cells by keying on surface molecules, often carbohydrates, on the plasma membrane

Traffic Across Membranes

• a steady traffic of small molecules and ions moves across the plasma membrane in both directions

for example, sugars, amino acids, and other nutrients enter a muscle cell and metabolic waste products leave

it also regulates concentrations of inorganic ions, like Na+, K+, Ca2+, and Cl-, by shuttling them across the membrane

• however, substances do not move across the barrier indiscriminately; membranes are selectively permeable

Permeability of a molecule through a membrane depends on the interaction of that molecule with the hydrophobic core of the membrane

• hydrophobic molecules, like hydrocarbons, CO2, and O2, can dissolve in the lipid bilayer and cross easily

• ions and polar molecules pass through with difficulty

• this includes small molecules, like water, and larger critical molecules, like glucose and other sugars

Specific ions and polar molecules can cross the lipid bilayer by passing through transport proteins that span the membrane

• some transport proteins have a hydrophilic channel that certain molecules or ions can use as a tunnel through the membrane

• others bind to these molecules and carry their passengers across the membrane physically

Each transport protein is specific as to the substances that it will translocate (move)

• for example, the glucose transport protein in the liver will carry glucose from the blood to the cytoplasm, but not fructose, its structural isomer

Passive transport

Diffusion is the tendency of molecules of any substance to spread out in the available space

• diffusion is driven by the intrinsic kinetic energy (thermal motion or heat) of molecules

In the absence of other forces, a substance will diffuse from where it is more concentrated to where it is less concentrated, down its concentration gradient

• the diffusion of a substance across a biological membrane is passive transport because it requires no energy from the cell to make it happen

• the concentration gradient represents potential energy and drives diffusion

Osmosis is the passive transport of water

• the solution with the higher concentration of solutes is hypertonic

• the solution with the lower concentration of solutes is hypotonic

Tap water is hypertonic compared to distilled water but hypotonic when compared to seawater

• solutions with equal solute concentrations are isotonic

Imagine that two sugar solutions differing in concentration are separated by a membrane that will allow water through, but not sugar

• the hypertonic solution has a lower water concentration than the hypotonic solution

• osmosis continues until the solutions are isotonic

Cell survival depends on balancing water uptake and loss

An animal cell immersed in an isotonic environment experiences no net movement of water across its plasma membrane

• water flows across the membrane, but at the same rate in both directions

The same cell in a hypertonic environment will lose water, shrivel, and probably die

A cell in a hypotonic solution will gain water, swell, and burst

Organisms without rigid walls have osmotic problems in either a hypertonic or hypotonic environment and must have adaptations for osmoregulation to maintain their internal environment

for example, Paramecium, a protist, is hypertonic when compared to the pond water in which it lives

• in spite of a cell membrane that is less permeable to water than other cells, water still continually enters the Paramecium cell

• to solve this problem, Paramecium have a specialized organelle, the contractile vacuole, that functions as a pump to force water out of the cell

The cells of plants, prokaryotes, fungi, and some protists have walls that contribute to the cell’s water balance

• a plant cell in a hypotonic solution will swell until the elastic wall opposes further uptake

• at this point the cell is turgid, a healthy state for most plant cells

Turgid cells contribute to the mechanical support of the plant

If a cell and its surroundings are isotonic, there is no movement of water into the cell and the cell is flaccid and the plant may wilt

In a hypertonic solution, a cell wall has no advantages

• as the plant cell loses water, its volume shrinks

• eventually, the plasma membrane pulls away from the wall

• this plasmolysis is usually lethal

Specific proteins facilitate passive transport of water and selected solutes:

Many polar molecules and ions that are normally impeded by the lipid bilayer of the membrane diffuse passively with the help of transport proteins that span the membrane

• the passive movement of molecules down its concentration gradient via a transport protein is called facilitated diffusion

• many transport proteins simply provide corridors allowing a specific molecule or ion to cross the membrane

Some channel proteins, gated channels, open or close depending on the presence or absence of a physical or chemical stimulus

for example, when neurotransmitters bind to specific gated channels on the receiving neuron, these channels open

• this allows sodium ions into a nerve cell

• when the neurotransmitters are not present, the channels are closed

Active Transport

Active transport is the pumping of solutes against their gradients

• some facilitated transport proteins can move solutes from the side where they are less concentrated to the side where they are more concentrated

• this active transport requires the cell to expend its own metabolic energy

• active transport is performed by specific proteins embedded in the membranes

(ATP)

The sodium-potassium pump actively maintains the gradient of sodium (Na+) and potassium ions (K+) across the membrane

• typically, an animal cell has higher concentrations of K+ and lower concentrations of Na+ inside the cell

• the sodium-potassium pump uses the energy of one ATP to pump three Na+ ions out and two K+ ions in

Some ion pumps generate voltage across membranes

• all cells maintain a voltage across their plasma membranes

The cytoplasm of a cell is negative in charge compared to the extracellular fluid because of an unequal distribution of cations and anions on opposite sides of the membrane

• two combined forces, collectively called the electrochemical gradient, drive the diffusion of ions across a membrane

• ions diffuse not simply down their concentration gradient, but diffuse down their electrochemical gradient

For example, before stimulation there is a higher concentration of Na+ outside a resting nerve cell

• when stimulated, a gated channel opens and Na+ diffuses into the cell down the electrochemical gradient

Special transport proteins, electrogenic pumps, generate the voltage gradients across a membrane

• the sodium-potassium pump in animals restores the electrochemical gradient not only by the active transport of Na+ and K+, but because it pumps two K+ ions inside for every three Na+ ions that it moves out

In plants, bacteria, and fungi, a proton pump is the major electrogenic pump, actively transporting H+ out of the cell

• protons pumps in the cristae of mitochondria and the thylakoids of chloroplasts concentrate H+ behind membranes

• these electrogenic pumps store energy that can be accessed for cellular work

Two Types of Active Transport

1. Exocytosis

2. Endocytosis

Small molecules and water enter or leave the cell through the lipid bilayer or by transport proteins

Large molecules, such as polysaccharides and proteins, cross the membrane via vesicles

During exocytosis, a transport vesicle budded from the Golgi apparatus is moved by the cytoskeleton to the plasma membrane

• when the two membranes come in contact, the bilayers fuse and spill the contents to the outside

During endocytosis, a cell brings in macromolecules and particulate matter by forming new vesicles from the plasma membrane

• endocytosis is a reversal of exocytosis

• a small area of the plasma membrane sinks inward to form a pocket

• as the pocket into the plasma membrane deepens, it pinches in, forming a vesicle containing the material that had been outside the cell

One type of endocytosis is phagocytosis, “cellular eating”

• in phagocytosis, the cell engulfs a particle by extending pseudopodia around it and packaging it in a large vacuole

• the contents of the vacuole are digested when the vacuole fuses with a lysosome

In pinocytosis, “cellular drinking,” a cell creates a vesicle around a droplet of extracellular fluid